Refactor cache_tree_update idiom from commit

We'll need to safely create or update the cache-tree data of the_index
from other places.  While at it, give it an argument that lets us
silence the messages produced by unmerged entries (which prevent it
from working).

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

merge-recursive.c

@@ -265,7 +265,7 @@ struct tree *write_tree_from_memory(struct merge_options *o)
 
 	if (!cache_tree_fully_valid(active_cache_tree) &&
 	    cache_tree_update(active_cache_tree,
-			      active_cache, active_nr, 0, 0) < 0)
+			      active_cache, active_nr, 0, 0, 0) < 0)
 		die("error building trees");
 
 	result = lookup_tree(active_cache_tree->sha1);